1. Field of the Invention
The present invention relates to a recording/reproducing apparatus for an information recording medium such as an optical disc or the like and to an aberration correcting unit and an optical pickup apparatus which are used for the recording/reproducing apparatus.
2. Description of the Related Art
There are optical discs such as a CD (Compact Disc) and a DVD (Digital Video Disc or Digital Versatile Disc) as well-known information recording media for optical recording and reproduction of information. Furthermore, the optical discs are of various types, for example, optical discs for reproduction only, write-once optical discs on which only additional recording can be done, and rewritable optical discs on which information can be erased and re-recorded. A recording medium having a plurality of recording layers on the same recording surface such as a 2-layer-per-side DVD is also being developed.
In addition, research and development are in progress for creating high-density optical discs and optical pickup apparatus and information recording and/or reproducing apparatus (hereinafter referred to as recording/reproducing apparatus) applicable to the high-density optical discs.
To cope with the high-density optical discs, there is a method of irradiating a laser beam of a small diameter onto the optical disc by increasing a numerical aperture (NA) of an objective lens provided in the optical pickup apparatus. Another method is the use of a light beam having a shorter wavelength.
However, the aberration of the light beam caused by an optical disc is increased when the numerical aperture NA of the objective lens is increased or a light beam having a shorter wavelength is used. This makes it difficult to improve accuracy of the information recording/reproduction performance.
For example, when an objective lens having a large numerical aperture NA is used, the amount of birefringence distribution which depends on the incidence angle is increased at the pupil surface of the optical disc, since the range of the incidence angle of the light beam to the optical disc is increased. This creates a problem of aberration due to the birefringence becoming more influential. In addition, when using an objective lens having a large numerical aperture and a light beam having a shorter wavelength, the influence of coma aberration is not negligible if the incident angle of the light beam to the normal direction (tilt angle) of the optical disc tilts at the time of recording or reproduction.
Although the aberration is also changed depending on a thickness of a transparent substrate (or transparent layer) through which the laser beam passes, since the thickness of a transparent substrate layer generally has an in-plane distribution, the aberration changes during the recording or reproduction of the optical disc. Particularly, the transparent layer thickness between the disc surface and the recording layer differs according to the recording layers in the optical disc having a plurality of recording layers on the same recording surface. Therefore, not only a magnitude of the aberration but also a distribution shape of the aberration in the plane perpendicular to an optical axis largely changes when changing the reading position or recording position from one recording layer to another.
An aberration correction liquid crystal unit having a liquid crystal such as a nematic liquid crystal or the like has been proposed in order to reduce the influence of the aberration upon recording or reproduction of the optical disc as mentioned above. There is disclosed a liquid crystal, for example, in the Japanese Patent Application Kokai H10-20263 as the liquid crystal unit for aberration correction as mentioned above.
FIG. 1 schematically shows an example of the aberration correcting unit. The aberration correcting unit has a structure such that a liquid crystal C such as a nematic liquid crystal or the like is held between transparent electrode layers A and B which are opposed to each other. The orientation state of the liquid crystal C can be changed by adjusting a voltage which is applied across the transparent electrode layers A and B. A birefringence change according to the orientation state is provided for a light when the light entering from one transparent electrode A (or B) passes through the liquid crystal C, and the light exits toward the other transparent electrode B (or A).
Furthermore, the transparent electrode layers A and B are formed in a divided manner, for example, each is divided into a plurality of transparent electrodes (a1, a2, and a3) and (b1, b2, and b3) . The transparent electrodes a1, a2, and a3 are electrically separated from each other, and the transparent electrodes b1, b2, and b3 are also electrically separated from each other.
The liquid crystal C can be adjusted so as to have a plurality of different orientation states therein by applying different voltages across the transparent electrodes opposing each other, for example, between the transparent electrodes a1 and b1, a2 and b2, and a3 and b3, respectively. Birefringence changes corresponding to the orientation states change within the liquid crystal C are simultaneously provided to the incident light. Thus, an aberration caused in an optical path can be corrected by properly adjusting the plurality of orientation states of the liquid crystal element.
As mentioned above, the aberration is changed depending on the in-plane position perpendicular to the optical axis due to the change in transparent layer thickness upon recording or reproduction. Additionally, the aberration is changed when changing the recording layer.
According to the conventional aberration correcting unit as mentioned above, the aberration caused in the optical path of an optical system is corrected by changing the voltage that is applied to each electrode of the liquid crystal. It is, however, difficult to correct the aberration at high speed upon recording or reproduction, since a response speed is low for a liquid crystal such as a nematic liquid crystal or the like which is used for aberration correction.